In oil and gas drilling data collected downhole from various sensors with respect to direction and orientation of the bit and/or geological formations encountered during drilling is transmitted uphole in various manners for use by the drilling operator at surface to control drilling.
Engaging in this practice is referred to as performing “Measurement-While-Drilling” (MWD). Logging data may also be transmitted uphole, in which case engaging in this practice is referred to as “Logging-While-Drilling” (LWD). Various types of telemetry technology exist to permit transmission of data collected downhole to the surface.
One type of commercialized telemetry technology is mud pulse telemetry. In mud pulse telemetry, pressure pulses are created downhole by periodically constricting the flow of the drilling mud through a drill string by means of a pulser actuator. Such pressure pulses then travel back through the drilling mud and/or the formation itself, where at surface such pressure pulses are received and decoded to reveal valuable information about drilling conditions downhole. When a main valve controlled by the pulser actuator is completely closed, mud flow through the valve is prevented and pressure in the drilling mud increases; when the main valve is subsequently opened, pressure in the drilling mud decreases. Pressure pulses can consequently be generated in the drilling mud by repeatedly opening and closing the main valve. One type of main valve used in pulsers is a poppet and orifice type valve, in which a poppet linearly reciprocates above a valve seat. When the poppet is pressed against the valve seat, the orifice and consequently the main valve are closed; the orifice and main valve are otherwise considered fractionally or entirely open. The pulser is typically housed within a tubular and attached to the bottom hole assembly (BHA) of a drill string when in use.
In some mud pulse telemetry units, a servo-valve having a pilot or poppet-type valve is used to actuate a main valve that is responsible for generating the sizable pressure pulses sent to the surface. The pilot/servo-valve is linearly oscillated by an electric motor, which poppet valve thereof is accordingly controlled to open and close at precise times and durations to thereby control pressure applied for similar times and durations to a main valve by drilling mud. By regulating the pressure on the main valve, the pilot valve can cause the main valve to open and close, at precise times and for varied duration, thereby generating the pressure pulses to specific intensity, duration, and sequence. In such manner data can be transmitted uphole and received at surface by sensors which receive such pressure pulses and decode such pressure pulses into useful information regarding conditions downhole during drilling.
It can be very useful to know, for a controller which controls the servo valve motor and thus the movement of the poppet valve to create pressure waves of various duration and intensity, the precise position of the pilot (servo) valve relative to the valve seat, namely whether the pilot valve (poppet) is in a fully open, fully closed, or some intermediate position therebetween. However, frequently LCM (i.e. lost circulation material, such as fine drill cuttings) sometimes enters the mud pulser unit and may impede or restrict to varying degrees the linear oscillation of the poppet, which may require the controller to initiate a “clearing” cycle to initiate a “back and forth” motion to the poppet valve to attempt to clear an LCM obstruction which otherwise prevents full open or full close positioning of the poppet. Knowledge of precise positioning of the poppet may then become unknown to the controller.
One way knowledge of the precise position of the poppet can be regained is by driving the poppet into or away from the valve seat until the poppet is physically restrained from further movement, typically by abutting a mechanical “stop” which limits further travel. When the poppet is so restrained the controller can detect that the motor draws increased current due to meeting the increased resistance of the “stop”, and can thereby determine the poppet has reached the limit of its travel in that direction, and thereafter cease driving the poppet in that direction. Driving the poppet in this manner is referred to as “overdriving” the poppet.
However, it is usually very undesirable to rely on repeatedly overdriving the poppet, after for example a cleaning cycle, to determine its position relative to the fully open or fully closed position because the current spikes that overdriving produces can relatively quickly drain the batteries that power the motor. Maintaining battery life of a downhole mud pulser is an extremely important consideration in increasing the downhole time of such pulser actuator. Otherwise, if batteries which power the pulser-actuator are “drained” more frequently, such causes a drilling operator to have to more frequently “trip out” a drill string to replace the mud pulser and/or batteries therein, which results in greatly-increased drilling expense, not to mention lost time in drilling, which adds further expense considering drilling rigs are generally rented to drilling companies on a per diem basis and lost time therefore results in increased equipment rental costs, to say nothing of the delay and lost profits caused in prolonging the time before revenue can be received from a well.
One example of a mud pulser/pulser actuator having a pilot valve used to actuate a main valve is given in U.S. Pat. No. 7,564,741 to Pratt et al. In Pratt et al., the pilot valve comprises a poppet and a valve seat, and a stepper motor is used to linearly move the poppet relative to the valve seat. A controller is connected to and controls the stepper motor. A plurality of Hall sensors (typically three for a three-pole stepper motor) are needed, which Hall-effect sensors are mounted on the stator of the stepper motor-output from such Hall-effect sensors allows the controller to “count” the number of rotations the motor's rotor undergoes. Counting the number of rotations of the motor, which moves the poppet through known reduction gearing, allows the controller to determine the linear position of the poppet relative to the valve seat. For example, the controller may first zero the poppet by forcing it against the valve seat, following which the motor can open the pilot valve by lifting the poppet a certain linear distance away from the valve seat (i.e. by the controller directing the motor to rotate a number of revolutions). This linear distance may be encoded in the controller as being a function of the number of rotations of the motor's rotor, as determined from measurements obtained using the Hall sensors on the motor, instead of in more conventional units such as inches or millimeters. Once the poppet is “zeroed”, by overdriving the poppet it need not ever be re-zeroed, and thus battery life of the mud pulser is preserved to a degree.
The mud pulser of Pratt is but one partial solution to the problem of continually determining the position of the poppet valve, while avoiding frequently having to overdrive the poppet to determine its position with the consequent resulting degradation in battery life.
Given the importance of MWD and LWD to the oil and gas industry in being best able to control drilling of wells, providing alternate mud pulser designs is important in providing alternate competing designs for facilitating completion in the industry
The foregoing background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information, or the reference in the drawings to “prior art” constitutes relevant prior art against the present invention.